WHAT IS THE TECTONIC RELATIONSHIP OF THE MIDDLE ORDOVICIAN BLOUNTIAN FORELAND BASIN TO THE TACONIC OROGENY IN THE SOUTHERN APPALACHIANS?

The Middle Ordovician foreland Blountian (Tellico-Sevier) clastic wedge has traditionally been interpreted as the primary manifestation of the earliest phase of the Taconic orogeny (i.e. Blountian) in the southern Appalachians, or even a separate and distinct tectonic event predating that orogeny. The tectonic setting of the Blount basin (BB), however, still remains open to debate, being attributed to either an arc obduction event and collisional orogen, similar to that recognized in the New England Taconic foreland, or a back-arc (retro-arc) environment associated with extensional tectonics and accretionary orogenic processes on the overriding, Laurentian plate. Reevaluation of existing data, including new constraints on the age of deformation and metamorphism along the western flank of the southernmost Appalachians, recent studies in the adjacent hinterland, and palinspastic reconstructions, suggest the BB is likely a retro-arc basin associated with B-type subduction and back-arc tectonics on the Laurentian outer margin. Interpretation of the BB as a retro-arc basin is based upon: A) compositional and structural contrasts with the central and northern Appalachian Taconic basins interpreted to have formed via A-Type subduction, B) the presence of Middle to Late Ordovician, Laurentian-affinity volcanic/sedimentary arc and back-arc basin successions within the Blue Ridge and Inner Piedmont that formed concurrently with, but palinspastically SE of the BB, C) the presence of large explosive felsic ash flow sheets and hypabyssal intrusives in the more distal back-arc sequences which may be the source of similar age K-bentonites within the BB, and D) elimination of the frontal Alleghanian metamorphic allochthon (western Blue Ridge/Talladega belt) as a potential source terrane for metamorphic and basement rock fragments. The possible presence of older, rift-related margin and intraplate basement faults in the middle and proximal foreland, possibly reactivated during Middle Ordovician subduction-related stress regimes, may have played a significant role in creating marginal uplifts that led to subsidence and filling of the BB in an accretionary, retro-arc tectonic setting.